Method and apparatus for increasing natural convection efficiency in long heat sinks

ABSTRACT

An electrical apparatus that is cooled via natural convection includes an electrical component, a vertical heat dissipation surface in thermal communication with the electrical component, and a diverter extending from the heat dissipation surface. The diverter disrupts vertical airflow over the heat dissipation surface.

This application is a continuation of U.S. application Ser. No.10/931,506, filed Sep. 1, 2004, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

Heat sinks are used to cool electrical devices, such as chips, diodes,and the like. Air, or some other fluid, flows over heat dissipationsurfaces of the heat sink to cool the heat sink, and thus the electricaldevice. The heat transfer is mainly by way of convection.

Generally, natural convection is the cooling of a vertical hot surfacein a large quiescent body of air. Lower-density air next to the verticalhot surface moves upward because of the buoyant force of higher-densitycooler air farther away from the vertical surface. Movement of the airadjacent the vertical surface increases in velocity from zero at thevertical surface to a maximum velocity at a distance from the verticalsurface and then decreases back to a velocity of zero as ambientsurrounding conditions are reached. The temperature of the moving airdecreases from the heated surface temperature to the ambient airtemperature. As the temperature of the moving air approaches theambient, the velocity at which the air moves approaches zero. No heatflows, by conduction or convection, where the velocity and temperaturegradients equal zero, thus this outer edge is referred to as theboundary layer.

Forced convection, where air is blown across a heated surface, resultsin a maximum air velocity at the outer edge of the boundary layer. Thedifference in the velocity profile and the higher velocities providemore air near the heated surface and very thin boundary layers. Naturalconvection forces are present; however, the natural convection forcesare negligible.

Forced convection may remove more heat than natural convection, butforced convection requires a device to move the air. In small electronicpackages or where it is desirable to minimize the amount of energyexpended to cool the electronic components, forced convection may beundesirable.

In free or natural convection, a tall heat sink tends to mix airinadequately with the surroundings resulting in the local ambienttemperature around the heat sink to be hotter at the top of the heatsink as compared to the bottom of the heat sink, especially in laminarflow regimes. With reference to FIG. 1, a long, tall vertically-orientedheat sink 10 creates natural convection by heating the air around heatsink fins 12. This air then rises, creating an airflow that removes heatfrom the heat sink 10. For the section near the top of the heat sink,the local ambient air temperature is warmer than the air entering at thebottom of the heat sink 10. Although the heat sink is removing heatgenerated by electrical components 14 (FIG. 2) by natural convection,this local temperature rise near the top of the heat sink can haveadverse effects on cooling, thus resulting in adverse effects on theelectronic components mounted near the top of the heat sink.

SUMMARY OF THE INVENTION

An electrical apparatus that is cooled via natural convection includesan electrical component, a vertical heat dissipation surface in thermalcommunication with the electrical component, and a diverter extendingfrom the heat dissipation surface. The diverter disrupts laminarvertical airflow over the heat dissipation surface.

A method for cooling an electrical component using natural convectioncomprises the following steps: attaching an electrical component to avertically oriented heat sink having a heat dissipation surface,introducing power into the electrical component such that the electricalcomponent heats the heat sink generating upward laminar flow via naturalconvection over the heat dissipating surface, and disrupting the upwardlaminar flow over the heat dissipating surface.

A heat sink for an electrical component includes a heat sink body and adiverter. The heat sink body is adapted to have an associated electricalcomponent mounted to the heat sink body, and the diverter is in thermalcommunication with the heat sink body. The diverter includes a firstcurved surface shaped to direct airflow toward the heat sink body and asecond curved surface shaped to direct airflow away from the heat sinkbody.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a rear perspective view of a known heat sink in thermalcommunication with an electrical component.

FIG. 2 is a front perspective view of the heat sink of FIG. 1 showingthe electrical components mounted to the heat sink.

FIG. 3 is a close-up schematic view of airflow over a fin of the heatsink of FIG. 1.

FIG. 4 is a rear perspective view of a heat sink including a pluralityof diverters.

FIG. 5 is a side view of a heat dissipation surface of a heat sinkhaving diverters mounted on the surface.

FIGS. 6A-6D are views of the diverter of FIG. 5.

FIG. 7A-7F are some alternative embodiments of diverters.

FIG. 8 is a schematic view of a refrigerated compartment having a heatsink.

FIG. 9 is an upper perspective schematic view of a diverter attached toa heat sink.

FIG. 10 is a lower perspective schematic view of a diverter attached toa heat sink, with the heat sink and an electrical component shown inphantom.

FIG. 11 is a perspective view of a model of airflow through the diverterof FIG. 9.

DETAILED DESCRIPTION

With reference to FIG. 4, a heat sink 20 for cooling electricalcomponents, for example LEDs, (not shown but similar to components 14 inFIG. 2) is provided. The heat sink 20 generally includes a base 22having a plurality of fins 24 extending from the base. Electricalcomponents can mount to the base on a mounting surface (not shown) thatis opposite the fins 24. Alternatively, the electrical components canmount to other surfaces that are in thermal communication with a heatdissipation surface 26. In this embodiment, the fins 24 along with therear side of the base 22 provide the heat dissipation surface 26 overwhich air flows to remove heat from the heat sink 20.

In this particular embodiment, a long, vertically oriented heat sink 22is provided to generate upward laminar flow via natural convection.Although not to be bound by theory, computer modeling has indicated thata long heat sink, i.e. a heat sink where the vertical dimension is aboutten times the remaining two dimensions, is capable of generating upwardlaminar airflow via natural convection on the order of one meter persecond when appropriate heating is applied. Nevertheless, not only areheat sinks where the vertical dimension is ten times the remaining twodimensions contemplated, shorter heat sinks, for example where thevertical dimension is about five times or even two times the remainingtwo dimensions, may achieve adequate results.

A plurality of diverters 28 mount to the heat dissipation surface 26 ofthe heat sink 20. The diverters 28 are vertically spaced from oneanother. The diverters 28 can also be horizontally spaced from oneanother. The mounting surface 26 can include designated mountinglocations 32 where the diverters 28 can attach to the heat dissipatingsurface 26. The mounting locations 28 can include holes that receivefasteners, or can simply include a marked location. According to theembodiment having mounting locations, the diverters 28 can beselectively removable from the heat dissipating surface 26.

With reference to FIG. 6, the diverters 28 are shown as ramps. Withreference to FIG. 5, the diverters 28 disrupt the upward laminar flow ofair across the heat dissipation surface 26 resulting in turbulentairflow adjacent the diverter. The turbulent airflow reduces the overalltemperature of the ambient air adjacent the heat sink by mixing incooler airflow located away from the heat sink. The diverter 28 can beshaped to create turbulent airflow that escapes the boundary layer oflaminar flow generated by natural convection. Accordingly, heat can bedissipated across the laminar flow bounder layer and mixing the coolersurrounding air with the hotter air adjacent the heat dissipationsurface 26. The diverters can also disrupt generally upward flow in theturbulent regime and the transitional regime. The diverters disrupt allthree kinds of flow moving air outside of the boundary layer to promoteheat transfer across the boundary layer.

With reference to FIG. 7 alternative configurations of diverters areshown. In some embodiments, the diverter includes a curved upstreamsurface. In other embodiments, the diverter comprises a thin plate likemember. Although many different configurations are disclosed in thefigures, the diverters can take any configuration that adequatelydisrupts the vertical flow.

With reference to FIG. 8, the heat sink 20 is situated in a refrigeratedcompartment 32. The heat sink 20 is long and narrow, similar to thosedescribed above, and typically mounts to a mullion of the refrigeratedcompartment 32. The refrigerated compartment 32 can include arefrigerator, a freezer, or other compartment typically found insupermarkets, convenience stores and the like. Lights 36 are used toilluminate the item stored on the shelves 34. The heat sink 20 includesa heat dissipation surface 26 and diverters 28. The diverters 28 canalso be similar to any of those described above.

With reference to FIGS. 9 and 10, another embodiment of a diverter 40 isshown. The diverter 40 attaches to and is in thermal communication witha heat sink 42 that has an electrical component 44 (FIG. 10) mounted tothe heat sink, similar to the embodiments disclosed above. The diverter40 of this embodiment exchanges relatively hot air with cool air bymoving outer cooler air toward the heat sink 42 and moving inner hotterair away from the heat sink.

The diverter 40 includes two generally parallel side walls 46 and 48that are spaced from one another. The side walls 46, 48 extend away fromand are generally perpendicular to the heat sink 42. A top or outer wall52 connects the side walls 46 and 48 and is generally parallel to andspaced from the surface of the heat sink 42 to which the divertermounts. The top wall 52 and side walls 46, 48 confine air as it movesthrough the diverter 40.

The diverter 40 also includes an upper or outer wall defining a firstcurved surface 54 that directs the flow of cooler outer air towards theheat sink 42 and a lower or inner wall that defines a second curvedsurface 56 that directs the flow of hotter inner air away from the heatsink 42. In this embodiment, the movement of air through the diverter 40can be described as a 180 degree rotation, in that the cooler outer airis directed to occupy the space that would have been occupied by thehotter inner air and the hotter inner air is directed to occupy thespace that would have been occupied by the cooler outer air, had therebeen no diverter.

The exact shape of the curved walls 54 and 56 is not critical. Thecurved walls are shown as emanating at a common line at a verticalmidpoint between the heat sink 42 and the top wall 52 along the sidewalls 46 and 48 of the diverter; however, the curved walls can emanateat any location along the side walls. The curve of the curved walls 54and 56 is simply to encourage the mixing of hot and cold air.

With reference to FIG. 11, airflow through the diverter (which is notdepicted in FIG. 11 for reasons of clarity) is shown. As can be seen,outer air particles that are spaced from the heat sink 42 move towardthe heat sink and inner air particles that are spaced adjacent the heatsink move away from the heat sink. The airflow returns to laminar flowdownstream from the diverter 40.

It has been found that an appropriately dimensioned heat sink, whenappropriate heating is applied, can generate enough air velocity simplyvia natural convection to incorporate the benefits of providingdiverters on the heat sink so that the need for fans, and the like, topromote airflow can be obviated. The heat sink has been described withreference to preferred embodiments; obviously alterations will occur toothers upon reading and understanding the detailed description. Theinvention is not limited to only those embodiments described in thepreceding description; the invention is only limited to the appendedclaims and the equivalents thereof.

1. A heat sink for an electrical component comprising: a heat sink bodyconfigured to have an associated electrical component mounted to theheat sink body, the body including a heat dissipation surface having avertical dimension substantially greater than a horizontal dimension;and a diverter extending from the heat dissipation surface, the diverterbeing positioned to disrupt upward vertical airflow that is generatedvia natural convection over the heat dissipation surface, the diverterbeing shaped to generate turbulent airflow near the heat dissipationsurface.
 2. The heat sink of claim 1, wherein the diverter is configuredto disrupt a laminar regime of the upward vertical airflow.
 3. The heatsink of claim 1, wherein the diverter is configured to disrupt aturbulent regime of the upward vertical airflow.
 4. The heat sink ofclaim 1, wherein the diverter is configured to disrupt a transitionalregime of the upward vertical airflow.
 5. The heat sink of claim 1,wherein the diverter is configured to promote heat transfer across aboundary layer of the upward vertical airflow.
 6. The heat sink of claim1, wherein the diverter is configured to generate an airflow that mixescooler air disposed away from the heat dissipation surface with hotterair disposed adjacent the heat dissipation surface.
 7. The heat sink ofclaim 1, wherein the vertical dimension is at least about 10 times afirst horizontal dimension of the heat dissipation surface.
 8. The heatsink of claim 7, wherein the vertical dimension is at least about 10times a second horizontal dimension of the heat dissipation surface. 9.A device comprising: a heat sink including a heat dissipation surface;an LED mounted to and in thermal communication with the heat sink, theLED being configured on the heat sink to provide sufficient heat to theheat sink to generate upward airflow via natural convection across theheat dissipation surface; and a diverter disposed with respect to theheat sink such that the diverter disrupts the upward airflow across theheat dissipation surface to mix cooler air that is spaced from the heatdissipation surface with hotter air that is adjacent the heatdissipation surface.
 10. The device of claim 9, further comprising aplurality of LEDs mounted to and in thermal communication with the heatsink, the LEDs being configured on the heat sink to provide sufficientheat to the heat sink to generate upward airflow via natural convectionacross the heat dissipation surface.
 11. The device of claim 9, whereinthe heat sink has a vertical dimension substantially greater than ahorizontal dimension.
 12. The device of claim 11, wherein the verticaldimension is at least about 10 times a first horizontal dimension of theheat dissipation surface.
 13. The heat sink of claim 12, wherein thevertical dimension is at least about 10 times a second horizontaldimension of the heat dissipation surface.
 14. A method for cooling anelectrical component using natural convection, the method comprising:attaching an electrical component to a vertically oriented heat sinkincluding a heat dissipation surface; introducing power into theelectrical component such that the electrical component heats the heatsink generating upward air flow via natural convection over the heatdissipating surface; and absent forced convection, disrupting the upwardair flow over the heat dissipating surface to mix cooler air spaced fromthe heat dissipation surface with hotter air disposed adjacent the heatdissipation surface.
 15. The method of claim 14, wherein the disruptingstep further comprises creating turbulent airflow that escapes aboundary layer of the upward air flow.
 16. The method of claim 14,wherein the disrupting step further comprises creating turbulent flowadjacent the heat dissipating surface.
 17. The method of claim 14,wherein the disrupting step further comprises disrupting the upward airflow in a turbulent regime of the upward air flow.
 18. The method ofclaim 14, wherein the disrupting step further comprises disrupting theupward air flow in a transitional regime of the upward air flow.
 19. Themethod of claim 14, wherein the disrupting step further comprisesdisrupting the upward air flow in a laminar regime of the upward airflow.
 20. The method of claim 14, wherein the disrupting step furthercomprises disrupting the upward air flow by passing air over a diverter.